Embodiments of the present invention relate to a receiver for receiving an analog signal, like a high-frequency signal, having a frequency band to be digitalized.
Such a receiver may, for example, be employed as a radio receiver or measuring receiver, in particular when radio or measuring signals are to be detected in high quality over a relatively high bandwidth. In a frequency range having a great frequency width, like in the frequency range below 30 MHz, high powers may occur at the antenna output, in particular with large receive antennas, depending on the conditions of propagation. Typically, this means a limitation of the bandwidth at the input of the receiver. Additionally, so-called intermodulation products of different orders may form. Second-order intermodulation products are particularly critical.
There are already some known approaches, like using filter stages at the input of the radio receiver. This is referred to as pre-selection and aims at preventing false reception, like at the so-called mirror frequency, or limiting the ingoing power. This approach is employed in both classic analog receive architectures, like superheterodyne receivers, and receivers where intermediate frequencies are digitalized.
In high-quality receivers, the relative bandwidth of the pre-selection filter advantageously is selected to be smaller than 1:2. This is referred to as the so-called sub-octave filter criterion. Using such sub-octave filters, it is possible to reduce second-order intermodulation products which may form in downstream stages.
In particular with low receive frequencies (like short waves of up to 30 MHz) and digital receivers, pre-selection is considerably narrower than the real-time bandwidth which would be possible from the point of view of a downstream analog-to-digital converter employed. This restricts the usability of the receiver considerably.
One prior-art approach is either switching the pre-selection filter in correspondence with the desired receive frequencies or adjusting the bandwidth of the filters to the receive conditions. Switching is, for example, discussed in EP 2 377 249 B1 using a radio signal receiver. Additionally, EP 2 191 579 B1 shows a device and a method for receiving an information signal having an information signal spectrum where the bandwidth of the filters are adjusted in correspondence with receive conditions. Due to this adjustment or, in particular, with bandwidths of greater than 1:2, however, the advantages for second-order intermodulation products vanish so that only the signal energy can be reduced. Therefore, there is need for an improved approach.
The object underlying the present invention is providing a concept which allows receiving and digitalizing an analog signal, comprising a frequency band to be digitalized having a large bandwidth, while avoiding or reducing intermodulation products (like of second order).